CLINICAL CASES
27
DNA and histones are synthesized to duplicate the chromosomes; and
G2 phase, during which there is cell growth and synthesis of macro-
molecules. Under certain conditions, the cell can enter a quiescent, or
G0 phase, which is not part of the regular cell cycle.
Dihydrofolate reductase (DHFR):
The enzyme required to convert folic
acid to its active form, tetrahydrofolate. It requires the cofactor NADPH
as a source of reducing equivalents to reduce folate first to DHFR and
then to tetrahydrofolate.
Methotrexate:
A drug that has a similar structure to DHFR. It binds to
DHFR reductase and competitively inhibits it, thus decreasing the levels
of tetrahydrofolate in the cells. It effectively stops DNA synthesis in rap-
idly dividing cells such as cancer cells.
Tetrahydrofolate (THF):
The active form of the vitamin folic acid. THF is
one of the major carriers of one-carbon units at various oxidation states
for biosynthetic reactions. It is required for the synthesis of the
nucleotide thymidylate (dTMP). Although bacteria can synthesize folic
acid, eukaryotes must obtain folate from the diet. Dietary sources of
folate include leafy green vegetables (e.g., spinach and turnip greens),
citrus fruits, and legumes. Many breakfast cereals, breads, and other
grain products are fortified with folate.
DNA:
Two large molecules composed of deoxynucleotides attached by
hydrogen bonds in a helical, antiparallel relationship.
Nucleosides:
Nitrogenous base plus a sugar (bases in DNA are adenine
[A], thymine [T], cytosine [C], and guanine [G]; bases in RNA are A, C,
G, but uracil [U] instead of T; sugar moiety in DNA is deoxyribose, and
in RNA is ribose).
Nucleotide:
Nucleoside plus phosphate group (see Table 3-1).
Deoxythymidylate (dTMP):
DNA nucleotide consisting of the deoxyri-
bose sugar, thymidine as nitrogenous base, and one phosphate.
Table 3-1
TERMINOLOGY OF NUCLEIC ACIDS
Bases
Purine (Adenine [A], Guanine [C])
Pyrimidine (Uracil [U], Cytosine [C], Thymine [T])
Sugar
Deoxyribose (DNA), Ribose (RNA)
Base + Sugar
Nucleoside
Base + Sugar + phosphate
Nucleotide
previous page 41 Case Files   Biochemistry read online next page 43 Case Files   Biochemistry read online Home Toggle text on/off